Today's mobile devices require integration of an ever increasing number of radio functions and frequencies (Bluetooth, WiFi, GNSS, GSM, 3G, LTE) over a proliferating number of bands (40+ for LTE alone). Most antennas that are designed to cover a very wide frequency range are generally referred to as broadband antennas. Broadband antennas are generally inefficient due to their wide frequency coverage, whilst higher efficiency narrow band antenna do not cover the required frequency range. Size is a further constraint which diminishes efficiency due to the drive to more compact antennas, and this also introduces severe isolation issues especially for MIMO. Mobile device antennas need to be efficient in order to conserve battery life and maintain coverage. Current implementations use up to six antennas to overcome this trade-off with the penalty of cost, size and lack of flexibility.
There are many proposals for reconfigurable antenna designs which would help to alleviate this problem. It is known to provide a reconfigurable antenna such as described in WO 2011/048357 (the content of which is incorporated into the present disclosure by reference) which has an extremely wide tuning range. However, this antenna is only able to access two services simultaneously. For example, the antenna can only support DVB-H (470 MHz) and GSM (900 MHz) signals or DVB-H (470 MHz) and WiFi (2400 MHz) or GSM (900 MHz) and GPS (1500 MHz) but it cannot support more than two of these services simultaneously, as required by current mobile devices which can require simultaneous access to GSM, GPS and WiFi. Furthermore, this particular antenna is unlikely to be adequate for future Cognitive Radio systems which will require multi-resolution spectrum sensing.
If multi-services or multi-spectrum sensing is required in the future then one solution would be to use more reconfigurable antennas. However, as mentioned above, providing multiple antennas in a small device is impracticable and so the system designers still need to address the problem concerning the small amount of space available to provide such services.
It is known from WO 2013/014458 (the content of which is incorporated into the present disclosure by reference) to provide a multi-output antenna in which each radiating element of an antenna device is connected to at least two matching circuits, and wherein each matching circuit is associated with a separate port arranged to drive a separate frequency such that each radiating element is operable to provide multiple outputs simultaneously. The separate frequencies may be adjusted independently of each other as required by adjusting the respective matching circuits, and this can be done with good isolation between the ports thereby offering very wide operating frequency range with simultaneous multi-independent output operations. Thus, the multiple outputs/ports may have independent frequency control (i.e. when the resonant frequency of port one is changed, the resonant frequency of port two will be unaffected and will remain the same).
Accordingly, a single antenna of the type disclosed in WO 2013/014458 can mimic the output from multiple separate antennas, while occupying less space than that required for such multiple separate antennas. This also allows use of fewer radiating elements, thus also reducing the problems associated with the coupling of separate radiating elements when they are placed in close proximity. Furthermore, as the matching circuits may be permanently coupled to the radiating elements so that the ports can be operated simultaneously, this can negate the need for switches and other complex circuitry required in order to select or isolate a particular output.
However, there are circumstances where it is not appropriate to provide a plurality of separate ports for each individual radiating element.
It is known from US 2007/0241985 to provide a dual band TV antenna device having first and second matching circuits arranged in parallel branches. The first matching circuit is adapted for impedance matching in a first frequency band, and the second matching circuit is adapted for impedance matching in a second frequency band. An incoming RF signal received by the antenna device is split into first and second frequency bands by way of appropriate filters. A switch is provided so as to select one or other of the first frequency band and the second frequency band for connected to a TV receiver. Accordingly, the first and second frequency bands are not used simultaneously. An alternative embodiment, shown in FIGS. 12 to 14, dispenses with the switch, and instead provides an additional filter between each matching circuit and the output port. However, as is made clear at paragraphs [0077] to [0080] and [0083] to [0084] of US 2007/0241985, only one of the two frequency bands is ever processed at any particular time. If the TV receiver is set to receive a channel in the FM/VHF band, then only the FM/VHF band signal is matched and tuned. Conversely, if the TV receiver is set to receive a channel in the UHF band, then only the UHF band signal is matched and tuned. There is no suggestion that the FM/VHF and the UHF bands are to be matched and tuned at the same time, with RF signals in each band being tuneable independently of those in the other band.